Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. In electrochemical fuel cells employing hydrogen as the fuel and oxygen as the oxidant, the reaction product is water. Recently, efforts have been devoted to identifying ways to operate electrochemical fuel cells using other than pure hydrogen as the fuel. Fuel cell systems operating on pure hydrogen are generally disadvantageous because of the expense of producing and storing pure hydrogen gas. In addition, the use of liquid fuels is preferable to pure, stored hydrogen in some mobile and vehicular applications of electrochemical fuel cells.
Recent efforts have focused on the use of hydrogen obtained from the chemical conversion of hydrocarbon and oxygenated fuels into hydrogen rich gas. However, to be useful for fuel cells and other similar hydrogen-based chemical applications, these fuels must be efficiently converted to relatively pure hydrogen with a minimal amount of undesirable chemical by-products, such as carbon monoxide (CO). The presence of such CO by-product greatly decreases the performance of the fuel cell and has a particularly detrimental effect on the anode of the fuel cell.
Conversion of hydrocarbons and oxygenated fuels such as methanol to hydrogen is generally accomplished through steam reformation in a reactor commonly referred to as a catalytic reformer. The steam reformation of methanol is represented by the following chemical equation: EQU CH.sub.3 OH+H.sub.2 O+heat.fwdarw.3H.sub.2 +CO.sub.2 (1)
Due to competing reactions and thermodynamic limitations, the initial gaseous mixture produced by steam reformation of methanol typically contains from about 0.5% to about 20% by volume of carbon monoxide and about 65% to about 75% hydrogen, along with about 10% to about 25% carbon dioxide on a dry basis (in addition, water vapor can be present in the gas stream). The initial gas mixture produced by the steam reformer can be further processed by a shift reactor (sometimes called a shift converter) to increase the hydrogen content and to reduce the carbon monoxide content to about 0.2% to about 2%. The catalyzed reaction occurring in the shift converter is represented by the following chemical equation: EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2 (2)
Even after a combination of steam reformer/shift converter processing, the product gas mixture will have minor amounts of carbon monoxide and various hydrocarbon species, each present at about 1% or less of the total product mixture. A variety of conventional treatment processes may be employed to remove many of the hydrocarbon impurities generated during the steam reformer/shift converter process. However, such conventional treatment methods are generally incapable of reducing the carbon monoxide content of the gases much below 0.2%. Although this fuel processing was described for methanol as the fuel is well known that other gaseous or liquid fuels, such as methane or gasoline, may be reformed to a hydrogen rich gas. Likewise alternatives to steam reforming, such as autothermal reforming, are also well known.
In low temperature, hydrogen-based fuel cell applications, which typically have an operating temperature of less than 100.degree. C., the presence of carbon monoxide in the inlet hydrogen stream, even at the 0.1% to 1% level, is generally unacceptable. In solid polymer electrolyte fuel cells, the electrochemical reaction is typically catalyzed by an active catalytic material comprising a noble metal, or noble metal alloys, such as platinum or platinum-ruthenium. In addition, other metals may be employed as a catalyst material, such as palladium or rhodium. Further, the noble metal may be promoted with metal oxides, such as iron oxide, cerium oxide, manganese dioxide, tungsten oxide, and the like.
However, carbon monoxide adsorbs preferentially to the surface of platinum, effectively poisoning the catalyst and significantly reducing the rate and efficiency of the desired electrochemical reaction. Thus, the amount of carbon monoxide in the hydrogen-containing gas mixture produced by a steam reformer/shift converter process for use in electrochemical fuel cells should be minimized, preferably to an amount significantly lower than the approximately 1% achieved using conventional steam reformation and shift conversion methods. The present selective oxidizing method and apparatus reduce the amount of carbon monoxide in a hydrogen-containing gas stream to a level suitable for use in low temperature electrochemical fuel cells, generally significantly less than 100 ppm.
In known selective oxidizing methods, it is believed that at least three competing reactions occur, which are represented by the following chemical equations:
1. The desired oxidation of carbon monoxide to carbon dioxide: EQU CO+1/2O.sub.2.fwdarw.CO.sub.2 (3)
2. The undesired oxidation of hydrogen to water: EQU H.sub.2 +1/2O.sub.2.fwdarw.H.sub.2 O (4)
3. The undesired reverse water gas shift reaction: EQU CO.sub.2 +H.sub.2.fwdarw.H.sub.2 O+CO (5)
One of the most common selective oxidizer designs uses an adiabatic catalyst bed to react the carbon monoxide with oxygen supplied by an oxygen-containing gas (e.g., air). Catalyst loading, bed space velocity, and air flow are selected to control the temperatures in the bed so that bed size is minimized while the selectivity of the reaction to consume carbon monoxide is maximized.
Performance of the selective oxidizer catalyst gradually decays due to the gradual poisoning of the catalyst active sites with carbon monoxide. After a period of time, this decrease in catalyst performance caused by carbon monoxide results in a rapid increase in the carbon monoxide concentration of the selective oxidizer exit gas stream which is fed as the inlet stream to the fuel cell assembly. In conventional selective oxidation methods, poisoning of the selective oxidizer catalyst by carbon monoxide can be compensated for by increasing the catalyst bed temperature. However, while an increase in the bed temperature helps to compensate for the loss of catalyst activity, it also results in the loss of reaction selectivity, and thus increased hydrogen consumption which is highly undesirable in fuel cell applications.
In view of the foregoing, it is understood in the art that regeneration of the selective oxidizer is required to reverse the gradual deterioration of catalyst performance due to the poisoning of the catalyst active sites. However, after the catalyst has been regenerated, the undesired process of poisoning of the catalyst will resume again. Thus, the need for periodic regeneration of the catalyst is required.
Prior art attempts have been made to address the concerns of poisoning of the catalyst as well as the need for periodic regeneration of the catalyst. There are particular problems associated with regeneration of the catalyst in that while the catalyst is being regenerated, the selective oxidizer cannot be used thus rendering it and, as a result, the fuel cell inoperative for a period of time. To address these problems, prior art systems have been developed where at least two selective oxidizers are arranged in series with one another and operating at two distinct temperatures. Alternatively, two selective oxidizer beds may be operated in parallel; with one being regenerated while the other operational. In that type of system, the first selective oxidizer operates at a temperature greater than the second to remove most of the carbon monoxide while maintaining a high enough temperature to eliminate poisoning effects.
However, the foregoing prior art systems are not well suited for automobile applications where size and weight are of paramount concern. As a result, the prior art systems with multiple selective oxidizer beds in series or in parallel suffer from the disadvantages of increased weight, volume, complexity and the associated cost. In addition, these systems operate at high temperatures thus requiring a separate heat exchanger to cool the stream prior to entering the fuel cell. Due to their high temperature and series arrangement, hydrogen fuel loss in the reaction is high resulting in poor over-all fuel cell efficiency.
In view of the foregoing, an improved catalyst and selective oxidizer bed regeneration process is desired that employ systems that are much lighter in weight than prior art systems and less complex in design and less expensive to manufacture, operate and maintain. Further, there is a desire for a process for regeneration of a selective oxidizer that employs only a single selective oxidizer bed in series with the fuel cell. There is also a desire for such a process to permit the selective oxidizer to operate at a relatively low temperature to obviate the need for a separate heat exchanger to cool the oxidized gas prior to the fuel entering the cell. It is also desirable that a suitable process for catalyst regeneration in a automobile environment be provided that is less expensive, more efficient and less expensive to manufacture and operate.